Light, breathable fabrics — cotton and linen over synthetics — handle the daytime heat far better, and comfortable closed shoes matter for walking temple grounds on uneven stone. Dress modestly, especially away from resort areas, is standard travel-advisory guidance for Egypt as a whole, which is why it applies at every site stop, not just the more obviously religious ones.
What "modest" means in practice
Covering shoulders and knees is the practical baseline for temple and mosque visits. Swimwear is fine for the ship's pool deck but isn't appropriate off the boat — packing one or two set-aside outfits makes this easier than improvising each morning.
Evenings on board
Evenings are generally relaxed rather than formal — some ships hold a themed dinner night where dressing up a little is part of the fun, but daily attire the rest of the trip stays casual. See What Should You Pack for a Nile Cruise? for the full packing list beyond clothing.